Perennial aquatic or marsh herb, stout; stems 25–60 cm tall, mostly 3–5(–9) mm diam., sparsely pubescent, the hairs simple, usually crisped, 0.2–0.25 mm long, rarely glabrous, rooting at nodes in fully emergent plants. Leaves dimorphic; submerged leaves in whorls of 5–8, ovate, 10–40 mm long, pectinate, with 12–20 pinnae; emergent leaves in whorls of 6–9, broadly lanceolate, 5–20 mm long, shortly pectinate, uppermost ones terete. Bracteoles of male flowers lanceolate to ovate, 0.6–0.7 mm long, entire; bracteoles of female flowers smaller; flowers sessile, solitary. Male flowers: sepals 4, ovate to oblong, 0.6–0.8 mm long; petals 4, 2–2.6 mm long, cream to reddish-brown or purplish, caducous; stamens 8. Female flowers: sepals and petals absent; ovary 4-celled, styles filiform, reflexed, stigmas white. Fruit cubiform, yellow-brown to deep red; mericarps cylindric, c. 1 mm long, prickly with sharp papillae, tapering slightly, apex obliquely truncate. Flowers mainly Oct.–Apr.